1. Field of the Invention
The present invention relates to a switching power supply circuit with a soft-start function for gently raising an output voltage during a start-up time.
2. Description of the Related Art
FIG. 6 shows one example of a main part of the circuit structure of a switching power supply circuit. This switching power supply circuit 1 includes a transformer 2. A main switching element (e.g., MOSFET) Q is connected in series to a primary coil N1 of this transformer 2. An external input power supply E is connected in parallel to a circuit composed of the primary coil N1 and the main switching element Q connected in series. A secondary rectifying and smoothing circuit 3 is connected to a secondary coil N2 of the transformer 2, and a load 4 is connected to this secondary rectifying and smoothing circuit 3.
A control circuit 5 is connected to the main switching element Q. This control circuit 5 outputs an ON/OFF pulse signal (switching-control signal), as shown in FIG. 7, to the main switching element Q and controls the switch ON/OFF operation of the main switching element Q. With the switch ON/OFF operation of the main switching element Q based on the control operation of the control circuit 5, electric current flows from the input power supply E to the primary coil N1, which causes a voltage to be output from the secondary coil N2. The voltage output from this secondary coil N2 is rectified and smoothed in the secondary rectifying and smoothing circuit 3, and a DC voltage Vout that has been rectified and smoothed is output to the load 4.
The switching power supply circuit 1 is provided with a detection circuit 6 for directly or indirectly detecting the output voltage Vout output from the switching power supply circuit 1 to the load 4. A detection voltage Vs for the output voltage Vout of the switching power supply circuit 1 is added to the control circuit 5 from this detection circuit 6. The control circuit 5 controls the switch ON/OFF operation of the main switching element Q based on the detection voltage Vs so that the output voltage Vout is stabilized to a predetermined set normal-operation output voltage Vp, as shown in FIG. 7. For example, the control circuit 5 includes an error amplifier 8, a reference power supply 9, a comparator 10, and a triangular-wave generating circuit 11 and controls the switch ON/OFF operation of the main switching element Q by using the PWM method.
More specifically, the error amplifier (error amplifier) 8 amplifies and outputs the differential voltage between the detection voltage Vs output from the detection circuit 6 and a reference voltage Vref of the reference power supply 9. A comparator 10 compares a voltage Ve output from the error amplifier 8 with a triangular wave voltage S, as shown in FIG. 7, output from the triangular-wave generating circuit 11. The comparator 10 then outputs an ON signal of the pulse signal shown in FIG. 7, for example, while a voltage value of the triangular wave voltage S is equal to or below the output voltage Ve of the error amplifier 8. On the other hand, the comparator 10 outputs an OFF signal of the pulse signal while a voltage value of the triangular wave voltage S is above the output voltage Ve of the error amplifier 8. A pulse signal output from this comparator 10 is added to the main switching element Q as the switching-control signal.
The control circuit 5 with the above-described structure controls the output voltage Vout to decrease to the set normal-operation output voltage Vp when, for example, the output voltage Vout exceeds the normal-operation output voltage Vp. In other words, the control circuit 5 shortens the pulse width of an ON signal of the switching-control signal to be added to the main switching element Q. This causes the output voltage Vout to decrease towards the normal-operation output voltage Vp. In contrast, when the output voltage Vout falls below the normal-operation output voltage Vp, the control circuit 5 performs control such that the decrease in the output voltage Vout relative to the normal-operation output voltage Vp is compensated for. More specifically, the control circuit 5 widens the pulse width of an ON signal of the switching-control signal to the main switching element Q. This causes the output voltage Vout to increase towards the normal-operation output voltage Vp.
A sharp rise of the output voltage Vout when the switching power supply circuit 1 starts up causes a significantly large overshoot voltage compared with the normal-operation output voltage Vp to occur. This overshoot voltage is added from the switching power supply circuit 1 to the load 4. As a result of the overshoot voltage being applied, a failure, such as latch-up, may occur in the load 4, possibly preventing the load 4 from starting up smoothly.
For this reason, the switching power supply circuit 1 shown in FIG. 6 is provided with a soft-start circuit 130. This soft-start circuit 130 includes a switching element (e.g., transistor element) 14, a resistor 15, a soft-start operation power supply 16, and a time-constant circuit 19. The time-constant circuit 19 is realized by, for example, a series circuit composed of a resistor 17 and a soft-start capacitor 18.
The switching power supply circuit 1 is provided with a start-up circuit 20 connected to the base of the switching element 14. When the switching power supply circuit 1 starts up, a voltage is applied from the start-up circuit 20 to the base of the switching element 14. As a result, the switching element 14 is switched from a switch ON state to a switch OFF state.
While the switching element 14 is in a switch ON state, the soft-start operation power supply 16 is electrically connected to the ground through the resistors 15 and 17 and the switching element 14. When the switching element 14 is switched from a switch ON state to a switch OFF state, the soft-start operation power supply 16 becomes electrically connected to the soft-start capacitor 18 through the resistor 17. As a result, the soft-start capacitor 18 is charged with a time constant determined by the capacitance of the capacitor 18 and the resistance of the resistor 17. A charge voltage Vz for the soft-start capacitor 18 gradually increases over time during the start-up time, for example, as shown in FIG. 7.
This charge voltage Vz for the soft-start capacitor 18 is applied to the comparator 10 of the control circuit 5 as a soft-start voltage. During the start-up time, the comparator 10 produces the switching-control signal, as described above, based on the charge voltage (soft-start voltage) Vz for the soft-start capacitor 18 and the triangular wave voltage S of the triangular-wave generating circuit 11. More specifically, the comparator 10 starts output of the switching-control signal after the charge voltage Vz for the soft-start capacitor 18 has reached the minimum level (signal-output starting voltage) Vlow of the triangular wave voltage S. Thereafter, the comparator 10 produces the switching-control signal based on the charge voltage Vz for the soft-start capacitor 18 and the triangular wave voltage S, and outputs the switching-control signal to the main switching element Q. Once a normal operation period has been reached after the start-up time, the comparator 10 produces the switching-control signal based on the output voltage Ve of the error amplifier 8 and the triangular wave voltage S, and outputs the switching-control signal to the main switching element Q.
During the start-up time, the main switching element Q starts switching operation in response to the switching-control signal being applied. With this switching operation, the output voltage Vout is output from the switching power supply circuit 1. With a gentle increase in the charge voltage Vz for the soft-start capacitor 18, the pulse width of the ON signal of the switching-control signal gradually widens, as shown in FIG. 7. As a result, the output voltage Vout of the switching power supply circuit 1 gently increases, as shown in FIG. 7. In other words, the switching power supply circuit 1 performs a soft start with the soft-start circuit 130. See, for example, Japanese Patent No. 3394915.
For the structure of the switching power supply circuit 1 shown in FIG. 6, output of the switching-control signal from the comparator 10 to the main switching element Q is started after the charge voltage Vz for the soft-start capacitor 18 has reached the predetermined signal-output starting voltage Vlow (the minimum level Vlow of the triangular wave voltage S of the triangular-wave generating circuit 11). The switching power supply circuit 1 then starts output of the DC voltage Vout. In other words, despite that the switching power supply circuit 1 starts driving, the switching power supply circuit 1 does not output the output voltage Vout until the charge voltage Vz for the soft-start capacitor 18 reaches the signal-output starting voltage Vlow.
For the structure of the soft-start circuit 130 shown in FIG. 6, the delay time from when the switching power supply circuit 1 starts driving to when output of the output voltage Vout is started, and the output rising time from when the switching power supply circuit 1 starts output of the output voltage Vout until the normal-operation output voltage Vp is reached, are determined by a time constant for charging the soft-start capacitor 18 in the time-constant circuit 19. In short, the time constant of the delay time is equal to that of the output rising time (in other words, the time constant for charging the soft-start capacitor 18 is constant over the entire start-up period of time).
For this reason, the soft-start capacitor 18 is gently charged during the period of time from when the switching power supply circuit 1 starts driving until the charge voltage Vz for the soft-start capacitor 18 reaches the signal-output starting voltage Vlow, despite no output of the output voltage Vout. Therefore, the delay time (the period of time from when the switching power supply circuit 1 starts driving until output of the output voltage Vout is started) becomes longer.
Thus, a problem to be overcome is described as a long time from when the switching power supply circuit 1 starts driving until the output voltage Vout reaches the normal-operation output voltage Vp.